The [function](http://nodejs.org/docs/latest/api/stream.html#stream_transform_transform_chunk_encoding_callback) that gets the first chunk.

### firstChunk.ctor()

Instead of returning a [stream.Transform](http://nodejs.org/docs/latest/api/stream.html#stream_class_stream_transform_1) instance, `firstChunk.ctor()` returns a constructor for a custom Transform. This is useful when you want to use the same transform logic in multiple instances.